In general, a wireless channel environment shows a lower reliability than a wired channel environment, due to multi-path propagation, shadowing, wave attenuation, time-varying noise, inter-cell interference, etc. These are the major causes of a low data transmission speed, that is, a low data rate in wireless communication. Accordingly, in order to implement a high-speed wireless environment providing a high data rate, there is the greatest need to remove the causes.
As technology for removing the causes, a Multiple-Input Multiple-Output (MIMO) system has been proposed. The MIMO system is a representative example of a multiple antenna system. The MIMO system has an advantage in that it can increase the system performance without supplying additional power and spectrum.
Generally, a multiple antenna system supports a single user mode (hereinafter, referred to as a “SU mode”) and a multi user mode (hereinafter, referred to as a “MU mode”).
A multiple antenna system that supports the SU mode transmits signals to the same user through a plurality of transmission antennas, and a multiple antenna system that supports the MU mode transmits signals to a plurality of users through a plurality of transmission antennas.
The multiple antenna system that supports the MU mode has been proposed, in order to provide more improved performance than the multiple antenna system that supports the SU mode, while minimizing the number of antennas and complexity in hardware configuration. Also, the MU mode is focused on increasing a system transmission capacity using Spatial Division Multiple Access (SDMA) scheduling.
Meanwhile, communication systems have adopted various multiplexing schemes in order to increase frequency efficiency. Representative examples of such multiplexing schemes are code division multiplexing, frequency division multiplexing, time division multiplexing, etc.
The code division multiplexing is a spread-spectrum technology of spreading a signal to be sent to a wider frequency band than the corresponding frequency band. That is, in the code division multiplexing, digitalized data is divided into several frames, and the respective frames are transmitted in correspondence to codes.
The frequency division multiplexing is a method of segmenting the frequency band of a communication medium into a plurality of narrower frequency bands and simultaneously transmitting the segmented frequency bands. In this case, a receiving party can filter a desired frequency component and restore it.
The time division multiplexing is a method of transmitting signals with time differences over a single transmission path. In the time division multiplexing, time is partitioned into basic time durations called time slots, the time slots are combined into a predetermined size of frame, and each time slot is allocated to a specific location on a frame for each channel.
However, the multiple antenna system requires a method of transmitting signals using a radio resource flexibly while increasing frequency efficiency when transmitting a plurality of independent user signals through a single radio resource. Also, a method of flexibly adjusting transmission factors so that all users' requirements can be maximally satisfied in various transmission environments is needed.